데이스타 쿼크를 사용중 한쪽 귀퉁이에 다른 곳보다 조금 밝은 부분이 있어, 문의하니 점검을 위해 보내줘도 되지만, 자기 생각엔 문제 없다며  

아래와 같은 글을 보내주네요.

Here is a segment about our mica etalon crystals that you may find helpful in this issue. 

At the heart of the DayStar narrow bandpass filter is an Etalon crystal.   
This is a naturally grown crystal made of Mica, mined from the ground, sorted for clarity, cleaved by hand into a thin wafer. We use only the brightest transmission, highest grade mica in the world. 
It is then sorted again for homogeniety and coated with our proprietary coatings to produce the narrow bandpass performance of a Fabry Perot Etalon.   Because the wafer on which the coatings lies is an actual cleaved crystal, mined from the ground, each crystal will maintain microscopic variations from crystal to crystal. Those variations cause slight changes in the index of refraction of the substrate, and therefore, changes to the FWHM and/or CWL of the resulting etalon. 

While mica crystal is flat to the molecular level, the matrix of the crystal growth still has variation due to the abundance percentages of the various elements within that crystal.  
Therefore, across the aperture of the etalon crystal, we see some etalons which are more uniform in their behavior than others.  
Flawless crystals with incredible uniformity, when discovered and qualified, are reserved for a PE classification. 
PE grade uniformity of this scale is necessary for Universities and research groups who will perform quantitative scientific measurements of the sun and it's chromosphere, where variations in wavelength will affect their resulting data.  

A uniform etalon crystal of PE grade will transmit  an even Central Wavelength accurate to the following specification: 
"The mean peak wavelength of all 12 mm. dia. areas shall be within ± 0.05 ångström of the full aperture wavelength measurement.

An SE grade filter will transmit with a similar accuracy of ± 0.5 ångström over the similar area. 

Because 1Å is 1/6562 wave, this means that these variations are incredibly microscopic. 
An SE grade uniformity of 0.5Å will be accurate to the scale of 1/13,124 wave. 
PE grade uniformity must be accurate to the scale of 1/131,240 wave.   This is within the scale between the nucleus of an atom and the first electron!  
While operating within a bandpass of 0.5Å total bandpass range, that very small variation can be detected with variations in transmission due to narrower FWHM or in center wavelength (CWL) 

We have also recently found that PE grade uniformity in FWHM (bandpass) across the aperture of the filter will affect density variations in photographic applications.   For example, areas with sharp changes between 0.4Å and 0.45Å can result from  crystal growth changes of direction and create a "sheer line".   This is different than a steppe between layers of the mica.  
A steppe causes an immediate 10Å jump in CWL.    These flaws do not pass our screening process and are rejected in quality control process. 
Photographic applications are more sensitive to SE grade filters with FWHM sheer or non-homogeniety.  
Some very serious photographers who want the absolute highest imaging quality will consider the PE grade for their imaging appli cations.

Why don't we just manufacture all etalons to the PE Grade?  
Due to the scarcity of flawless, PE grade stock which qualifies at the scale of accuracy required, those PE grade etalon crystals are reserved for applications where the client has the research requirement and has the budget to cover the costs to grade, test, tune and qualify the substrate on the top end test equipment as required.  

Remember that 1 nanometer is 1/billionth of a meter. 
1Å is 0.1 nanometer.
So the accuracy of the of ± 0.05 ångström uniformity of the transmission of the PE grade filter equals 5/1,000,000,000,000 (5 / 1 trillionth ) of a meter.  This is 1/130,000 of a wave. 

We are proud to offer SE grade filters to the non-research based astronomy market for high quality, reasonably priced solar observing.  
SE grade etalon crystals are available in Quantum SE, Quark filters and SolaREDi telescopes. 

Kind regards,